This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Molybdenum is the only second-row transition element known to be essential for humans. It is required for a very large number of biological processes, and its importance is increasingly being realized. Since the publication of the first structure thirteen years ago. The current PDB contains some 171 entries containing molybdenum or molybdopterin in the molecule name (excluding the nitrogenase proteins), and multiple structures are now available for every major category and type of Mo enzyme. This wealth of structural information has revolutionized and considerably stimulated the field, but many open questions remain, particularly concerning the detailed structure of the molybdenum site and the nature of the catalytic mechanisms. The goal of our proposed program is to develop an in-depth quantitative structural and electronic basis for the catalytic mechanisms used by Mo enzymes, and to compare and contrast them between and within the different families. We will primarily employ x-ray absorption spectroscopy (XAS) to investigate the physical and electronic structure of the active sites, and in many cases we will apply a holistic approach, which will combine information from XAS, density functional theory (DFT), and available crystal structure data to provide the most accurate picture yet of the active site. This holistic approach is in principal applicable to any metal, so its development has broad implications for metalloenzyme studies.